1. Field of the Invention
The present invention relates to automatic calibration of clock circuits normally powered by alternating current of a uniform known frequency during periods of interruption.
2. Description of the Prior Art
Electric clock circuits have been utilized for many years to power timekeeping devices, such as electric clocks automatic ovens, automatic locks, and so forth. More recently, electric clock circuitry has been employed to govern periodic operations in thermostat heating and cooling control systems. For example, night setback thermostat systems have been developed for use in home and office heating and air conditioning to maintain a preset daytime temperature during certain hours of the day, and to thereafter maintain a lower temperature during the night. Such systems tend to conserve energy, since cooler temperatures are generally more desirable at nighttime. This is true with respect to buildings in which occupants sleep and in buildings which are vacated during the night, but within which a minimum temperature must be maintained to prevent damage to plumbing fixtures. The same type of timing circuitry is applied to allow an increase in temperature at night when operating an air conditioning system in which a building is vacated during the night hours. In either event, an automatic time keeping system is necessary to ensure that the appropriate set point temperature changes occur at the scheduled times.
Most conventional electrical timekeeping devices operate from commercially available alternating current provided by electrical utilities. This current is typically at 110 to 120 volts and is provided as a sinusoidal current of a fixed frequency, usually 60 hertz. There are some localities in which the cyclic frequency is 50 hertz. Whatever the frequency, within a given location frequency is maintained constant.
In the electrical timepieces driven by such alternating current, the current is provided to an electric motor to rotate a shaft carrying an armature at a rate determined by the frequency rate of the alternating current provided. Naturally, a temporary interruption in alternating current service removes the power input to such timekeeping devices, which thereafter remain dormant until power is restored. In such instances of power failure or interruption, the timepieces must be manually reset by reference to a clock not subject to the power failure.
More recently, some types of electrical timekeeping devices have been operated from direct current power sources. With the advent of minaturized LSI circuitry, electrical wrist watches and clocks and other chronometers are operated from direct current power supplies. In such devices, the d.c. power supply either operates a d.c. motor to turn a shaft and armature, or the d.c. power supply powers an oscillator, such as a high frequency crystal oscillator, to provide a series of timing pulses occurring at uniform intervals. Time pieces driven by electric d.c. motors are notoriously inaccurate since, as the voltage level of the d.c. power source deteriorates with use, the motor speed slows. This causes the electric timepiece to lose time, so that it must be continually corrected by manual compensation. Those systems employing crystal oscillators to produce timing signals likewise suffer from certain deficiencies. Specifically, as the d.c. voltage level falls with deterioration of the d.c. voltage source, a threshold level is reached below which the voltage and current available are insufficient to drive the oscillator. The result is, in effect, a power failure to the time keeping mechanism.